The present Invention concerns a method of manufacturing a semiconductor device and, more particularly, it relates to a method of manufacturing a semiconductor device capable of forming electrodes or wirings of excellent step coverage at a low temperature.
Polycrystalline silicon (Si) films formed by thermal decomposition of silane (SiH.sub.4) through a low pressure chemical vapor deposition (LPCVD) method have been generally utilized for electrodes and wirings in semiconductor devices. Since the polycrystalline Si film formed by the LPCVD method has extremely high resistivity, impurities are doped by well-known thermal diffusion or ion implantation to lower the resistivity and provide an electroconductivity in the subsequent step.
In the manufacture of a bipolar transistor, an opening reaching as far as an Si substrate is formed in a dielectric film formed on the Si substrate, a polycrystalline Si film is then deposited, to which impurities are doped by the ion implantation and, subsequently, the impurities in the polycrystalline Si film are diffused to the Si substrate by a heat treatment to form an emitter. The techniques relevant to the method of this kind, is discussed, for example, in VLSI Technology, S.M.Sze, ed (McGraw-Hill, 1988 pp 499-507).
However, in a case of forming the emitter of a pnp bipolar transistor by implantation of boron ions or boron difluoride (BF.sub.2) ions into the polycrystalline Si film, a heat treatment at a high temperature of higher than 900.degree. C. is required for activating boron in the polycrystalline Si film and boron of a large diffusivity is diffused over a long distance making it difficult to form a shallower junction. As a result, there has been a problem that the operation speed of the pnp transistor cannot be improved.
Further, along with the down scaling for LSI, the aspect ratio in the opening for forming the emitter is increased and, in a case of ion implantation to the polycrystalline Si film in the opening having such an abrupt side wall, a portion lacking in boron is formed in the polycrystalline Si film thereby bringing about a problem that the resistivity of electrodes or wirings is increased.
On the other hand, in a case of doping impurities by a thermal diffusion, it is possible to dope the impurities also on the steep stepped side walls by applying a thermal diffusion at a high temperature for a long period of time. However, similar to the case of the ion implantation described above, boron is diffused over a long distance in the portion of the Si substrate in which the polycrystalline Si film is in contact with an Si substrate such as an emitter, making it difficult to form a shallower junction.
As a method of doping boron to the polycrystalline Si film formed on the side walls of the opening as one of the foregoing problems, has also been proposed a method of using disilane (Si.sub.2 H.sub.6) and diborane (B.sub.2 H.sub.6) as source gases and depositing an Si film while doping boron within a temperature range of 520.degree. C.-665.degree. C. (J. Electrochem. Soc: SOLID-STATE SCIENCE AND TECHNOLOGY, Vol. 133, No. 8, pp 1721-1724, August 1986). However, according to the experiment made by the present inventors, the deposited Si film within such a temperature range by using Si.sub.2 H.sub.6 as the source gas brings about a problem that it is poor in the step coverage and the layer thickness on the side wall of the groove is remarkably reduced as compared with the upper surface upon deposition in a deep groove. Further, it has also been found that the deposition reaction of the film is too violent to be controlled within the temperature range of 400.degree. to 600.degree. C.